Traveling wave tubes are capable of amplifying and generating microwave signals over a considerable frequency range (e.g. 1-90 GHz) with relatively high output powers (e.g. &gt;10 megawatts), relatively large signal gains (e.g. 60 dB), and over relatively broad bandwidths (e.g. &gt;10%).
In a traveling wave tube, an electron gun generates a beam of electrons which are directed through a slow-wave structure and collected by a multi-electrode collector. A beam-focusing structure surrounding the slow-wave structure creates an axial magnetic field that contains the electron beam within the slow-wave structure. The slow-wave structure generally comprises either a helical conductor or a coupled cavity circuit with signal input and output ports located at opposite ends thereof, wherein a microwave signal applied to one of the ports propagates along the slow-wave structure to the other port at a projected axial velocity that is considerably less than the free space speed of light. With the velocity of the electron beam adjusted to be similar to the projected axial velocity of the microwave signal propagating along the slow-wave structure, the fields of the microwave signal and electron beam interact with one another so as to transfer energy from the electron beam to the microwave signal, thereby amplifying the microwave signal.
A traveling wave tube may be used as an amplifier by operatively coupling a microwave signal to be amplified to the signal input port of the slow-wave structure. The microwave signal propagates towards the signal output port in the same direction as the electron beam and becomes amplified by energy extracted from the electron beam. As a result of this energy exchange, the electron beam loses energy which reduces the velocity thereof.
A traveling wave tube may also be used as a backward-wave oscillator, wherein random, thermally generated noise interacts with the electron beam to generate a microwave signal in the slow-wave structure of the traveling wave tube. Energy is transferred to the microwave signal propagating along the slow-wave structure in a direction opposite to that of the electron beam, whereby the oscillator output signal is generated at the signal input port of the slow-wave structure, with the signal output port of the slow-wave structure terminated with a microwave load.
One problem with prior art traveling wave tubes is that the electrons are collected by collector electrodes in the multi-electrode collector that operate at respective potentials greater than or equal to the potential of the cathode. However, under certain conditions, particularly when a traveling wave tube is operated far below saturation (i.e. more than 10 dB), some of the electrons in the electron beam can have associated energies that are greater than the energy associated with the cathode potential. These relatively high energy electrons are a source of potentially recoverable energy that is not recovered by prior art traveling wave tube systems.